Micro-electro-mechanical system (MEMS) transducers such as MEMS microphones are increasingly finding application in portable electronic devices such as mobile telephones, laptop and tablet computers, audio and video players, personal digital assistants (PDAs) and wearable devices such as smart watches, at least in part due to the small size of such transducers.
Transducers such as capacitive microphones or pressure sensor devices formed using MEMS fabrication processes typically comprise an electrode that is moveable with respect to a fixed electrode in response to incident acoustic or pressure waves, such that the fixed electrode and the moveable electrode together form a variable capacitance. The moveable electrode may, for example, be supported by a flexible membrane. In use a first one of the electrodes may be biased by a relatively high, stable bias voltage VBIAS, which may be of the order of 12V, whilst the other electrode is biased to another fixed voltage VREF, typically ground, via a very high impedance, for example, of the order of 10 GΩ. Acoustic or pressure waves incident on the transducer will cause displacement of the moveable electrode with respect to the fixed electrode, thus changing the spacing between these electrodes and hence the inter-electrode capacitance. As the second electrode of the transducer is biased via a very high impedance, these changes in capacitance cause an output signal voltage to appear at an output terminal of the transducer. Given the small capacitance of the MEMS transducer this output signal voltage is relatively small and thus the output signal voltage is typically amplified by a low-noise amplifier (LNA) arrangement.
The spacing between the fixed electrode and the moveable electrode in an equilibrium position (i.e. in the absence of an incident acoustic or pressure wave) is typically of the order of 1-3 μm. Under normal operating conditions the displacement of the moveable electrode towards the fixed electrode in response to incident acoustic or pressure waves may be up to 30% of the average spacing between the fixed electrode and the moveable electrode in its equilibrium position, i.e. if the average spacing between the fixed electrode and the moveable electrode in the equilibrium position is 3 μm then the displacement of the moveable electrode towards the fixed electrode by an incident pressure wave may be up to 1 μm. Thus the spacing, i.e. air gap, between the moveable electrode and the fixed electrode during normal operation of a MEMS transducer may be as small as 2 μm at times.
A problem can arise if the MEMS transducer is subjected to an excessive sound pressure level (SPL) arising from, for example, use in a very loud environment or acoustic shock such as can occur if a device incorporating the transducer is tapped or dropped. In such circumstances the displacement of the moveable electrode may exceed its normal operating range, and the moveable electrode may as a consequence be electrostatically captured by the fixed electrode. In this captured state the sensitivity of the transducer is greatly reduced and audio is not properly captured by the transducer. There is also a risk of permanent mechanical stiction of the moveable electrode to the fixed electrode if the fixed electrode and the moveable electrode do not incorporate design features to mitigate the risk of contact. In the event that the moveable electrode is electrostatically captured by the fixed electrode, it is unable to return to its normal operating state until it is discharged, which typically only occurs after a supply voltage to the moveable electrode is disconnected.
Protection systems exist to protect the LNA arrangement from excessive input voltages that may arise as a result of an event that gives rise to an excessive SPL at the transducer. Such systems typically operate by detecting the event and disabling an input of the LNA arrangement for a predefined period of time. During the predefined period of time the moveable electrode of the MEMS transducer may be discharged, to release it from the fixed electrode if it has been electrostatically captured by the fixed electrode. At the end of the predefined period of time the input of the LNA arrangement is re-enabled and the moveable electrode is re-charged to its normal operating level, thus permitting normal use of the MEMS transducer and LNA arrangement.
One problem with protection systems of the kind described above is that there is no check, prior to re-enabling the input of the LNA arrangement and re-charging the moveable electrode to its normal level, if the SPL at the transducer has returned to a safe level at that time. Thus, it is possible that the input of the LNA arrangement will be re-enabled and the moveable electrode will be re-charged while an excessive SPL is still present at the transducer, leading to the moveable electrode being electrostatically captured again and the attendant risk of permanent stiction, as well as the required inoperative period of the transducer while the moveable electrode is discharged to release it from the fixed electrode and subsequently re-charged.
Accordingly, a need exists for a system that protects a MEMS transducer from electrostatic capture.